1. Field of the Invention
The present invention relates to a method and apparatus for sensing electrical voltage in a polyphase electrical system and more particularly to a method and apparatus for sensing changes in amplitude of the terminal voltage of a polyphase electrical generator.
2. Description of the Prior Art
Polyphase electrical generators, normally three phase, are generally used in the generation of electrical power in a utility system. As illustrated in FIG. 1, such generators 20 are normally driven by a prime mover 22, such as a steam, gas or water turbine, rigidly attached to the generator shaft to drive the generator rotor at a predetermined speed, which establishes the frequency of the generator output voltage, known as the terminal voltage.
Since the polyphase generator is normally connected to a transmission and distribution network, it is necessary to regulate the generator terminal voltage within a predetermined range. Accordingly, such generators are normally provided as separately excited generators with a separate generator field winding 24. As is known in the art, the magnitude of the terminal voltage is a function of the field current If applied to the generator field winding 24. Thus, in order to control the magnitude of the terminal voltage of a polyphase generator, it is necessary to control the magnitude of the field current I.sub.f applied to the generator field winding 24. Devices known as automatic voltage regulators (AVR's) are normally used to control the field current I.sub.f in order to regulate the terminal voltage. More specifically, such devices normally sense the phase voltages of an electrical generator by way of potential transformers 28 and compare the sensed phase voltages with a terminal voltage set point 30. Any difference between the sensed phase voltage and the terminal voltage set point 30 is used to create an error signal by closed loop control that is amplified, for example, by a power amplifier 32 and used to adjust the field current I.sub.f in the generator field winding 24 in order to minimize the error signal and regulate the terminal voltage of the generator 20.
A simplified block diagram of a known automatic voltage regulator 26 is illustrated in FIG. 2. In such a device, the phase voltages V.sub.A, V.sub.B and V.sub.C, as sensed by the potential transformers 28, are applied to a rectification and filtering circuit 34 in order to convert phase voltages, which are sinusoidal, to a suitable DC voltage that can be compared with the terminal voltage reference set point 30 by way of a comparator 36. The output of the comparator 36 defines an error signal which may then be combined with other signals by way of a signal mixing circuit 38 to control the field current I.sub.f in the generator field winding 24 to regulate the terminal voltage of the polyphase generator 20.
A known rectification and filtering circuit 34 is illustrated in FIG. 3. As shown, the rectification of the phase voltages V.sub.A, V.sub.B and V.sub.C is provided by six rectification diodes 40. In order to smooth out the output signal from the rectification circuit 34, the rectified voltage is then applied to a filtering circuit 42 which may consist of a resistor 44 and a capacitor 46.
A problem arises in selecting a time constant for the filtering circuit 42. As shown in FIG. 4, the filter time constant is related to the filter ripple amplitude. More specifically, filter circuits having relatively large filter time constants produce a relatively low amount of ripple but result in a relatively unacceptable response time. Conversely, providing relatively smaller filter time constants results in relatively unacceptable ripple amplitudes as shown in FIG. 5. Accordingly, it is relatively difficult to select a filter time constant for an AVR which provides a relatively small amount of ripple voltage and at the same time provide a acceptable response time.